There is not really a way to do this as such in FileMaker, one option is to use the 'GetThumbnail ( )' function FileMaker Pro 16 Help but this returns a 72 dpi image, so may not be an option if you want to retain a high quality image.

With FileMaker 16 you can use JSON API's to integrate with online tools that can do this for you potentially. A quick google search for free Image Conversion API came up with this service Zamzar - File Conversion API there are plenty more that have a fee